1. Field of the Invention
The present invention relates to optical measurement of turbid media and in particular to optical measurement tissue absorption and scattering parameters and tissue imaging. The invention may also be used as a means to monitor a chemical process, for example, a pharmaceutical slurry.
2. Description of the Prior Art
Reflectance spectroscopy is a technique for characterizing turbid media that has become widely used in medical diagnostics. In many cases access to quantitative information, for example, chromophore concentrations is desired. In the strictest sense, this may require the ability to separate the effects of absorption from those of scattering. However, it may be that calibrated, structured reflectance data may be sufficiently quantitative without going to the step of separating scattering from absorption. Fundamentally, the coefficient of absorption μa and the coefficient of reduced scattering μs′ can be determined by a series of reflectance measurements performed in one of three domains, namely, time (with a fast pulse of light), frequency (with a sinusoidally modulated source of light), and steady state (with a source of constant intensity but multiple detectors at different distances).
Unsurprisingly, these three techniques have different merits and limitations. Spatially resolved steady-state techniques are relatively inexpensive and are more readily suited for the determination of μa and μs′ over large, continuous ranges of wavelengths than are the other methods. However, the steady-state approach works best when measurements are performed with a combination of short (˜1 transport mean free path) and long (many transport mean free paths) source-detector separations. Ideally, the optical properties of the sample should not vary over the ranges of volumes probed by the various measurements. The larger the spread of distances probed, the more likely that heterogeneities, such as those found in biological tissue, will distort the data from the predictions of the model. One approach to limiting this effect, given that the shortest separations provide great stability for the calculation of μs′, is to use relatively short (<10 mm) source-detector separations. Inasmuch as the mean probing depth scales with the source-detector separation, with this approach such measurements are sensitive to superficial components (to depths of less than 5 mm for typical biological tissues).
Time-domain and frequency-domain techniques are well suited for deeper (>1 cm for biological tissue) investigations. Moreover, they can be performed with only one or a few source-detector separations, which makes them more robust for use in studying heterogeneous samples. Because such techniques require sources that can be pulsed or modulated rapidly, covering a large wavelength range requires a tunable laser or an extensive collection of laser diodes, both of which can be expensive, difficult to maintain, and slow to cover the entire spectrum. This is an important drawback, because the quantification of chromophore concentrations can be significantly affected by use of a limited number of wavelengths.
Thus, a means has been devised to use steady-state and frequency-domain reflectance measurements in tandem to obtain broad wavelength coverage with increased penetration depth. This method comprises performing frequency-domain photon migration measurements on a turbid medium sample. It also includes performing steady-state reflectance measurements on the same turbid medium sample. It further includes combining the results of the frequency-domain photon migration measurements with the results of the steady-state reflectance measurements for obtaining a unique spectrum for the turbid medium sample.
Light photons from a plurality of different laser diode light sources are emitted into human body tissue from a predetermined delivery point on the surface of a human body. In this method light photons are emitted from a white light source into the same human body tissue from the same predetermined delivery point on the surface of the human body. The method then includes collecting the light photons received at a spaced collection point on the surface of the human body after such light photons have traveled through the human body tissue intermediate the delivery and collection points. The predetermined characteristics of the light photons from the different light sources are then collected to provide an indication of the composition of the traversed human body tissue. The prior methodology is described in detail in U.S. Provisional Patent Application Ser. No. 60/308,507 filed on Jul. 27, 2001, entitled “Broadband Absorption Spectroscopy in Turbid Media By Combined Frequency-Domain and Steady-State Methods and Apparatus”, assigned to the same assignee as the present invention and is incorporated herein by reference.
Absorption and scattering properties provide unique insight into tissue function and structure. “Optical biopsy” techniques have been developed based on the quantitative measurements of such properties and have shown promising results in clinical trials. Nevertheless, most of these techniques as described above reply on “point spectroscopy” and measure only a single, small area of tissue at a time. Such investigations would greatly benefit from imaging capabilities and/or being performed over a larger region of interest. Scanning or multiplexing can be used to overcome such a disadvantage, but is typically slow and cumbersome to implement.
What is needed is some kind of methodology and apparatus whereby larger a real information or data relating to the absorption and scattering properties of tissue or turbid media may be rapidly acquired in a single measurement.